This invention relates to high-velocity gas burners having enhanced flame stability. More particularly, the invention relates to high-velocity gas burners having enhanced flame stability over wide operating ranges and are utilizable in melting furnaces and the like.
The need for burners providing efficient heating and melting is a very important industrial concern, especially today when energy costs are rising and supplies of fuel are decreasing. Along with this is the need for the burners to be adaptable for use in furnaces at both low and high tonnage operating conditions while maintaining the production of commercially acceptable molten metal products, i.e., free of contamination.
Gas burners of the type provided by the present invention may be used in several different furnaces or units where a high heat level is required. The present gas burners have been found to be effective in melting furnaces such as the upright melting furnace described and illustrated in U.S. Pat. No. 3,199,977 issued to Albert J. Phillips et al. on Aug. 10, 1965. In that patent, the gas burners are inserted into each of the side wall ports and held in position therein by bolts which hold the mounting plating of each burner body tightly against the shell of the furnace so as to provide a substantially gas tight mounting. This patent as well as other related patents does not suggest any means for enhancing the flame stability of the burner installed in a furnace or other highlevel heating unit. See, for example, U.S. Pat. No. 3,701,517; 3,715,203; 3,788,623; as well as the additional prior art cited within the reference. Several gas burner designs are disclosed in these references, and other burner combustion chamber systems have been discussed in U.S. Pat. Nos. 3,299,940; 4,211,555; 4,301,997; 4,309,170; and 4,311,519, Canadian Pat. No. 1,100,029 and DE Offenlegungsschrift No. 29 46 120.
A difficult problem in the burner art, especially with regard to their use in furnaces such as the Phillips et al. upright melting furnace, has been to provide a stable flame over wide operating conditions. In a burner of the configuration of the present invention, the flame will burn both inside and outside of the combustion chamber and its shape in the chamber is defined by the shape of the chamber, with its shape outside the chamber being generally conical. The burners typically have an igniter bar in the ignition section with the combustion chamber having a wider diameter than the outlet of the igniter section. The flame is held immediately downstream of the igniter bar and spreads to the unburned mixture passing by. Additional flame-holding is obtained with the annular area provided by the shoulder at the juncture of the igniter section and combustion chamber. Thus, two flame fronts are established, one spreading out from the igniter bar and the other spreading from the annular shoulder type flame-holder.
Unfortunately, however, the flame spreading from the annular shoulder may be unstable in that it does not completely form on the shoulder and, consequently, forms a black or cold spot at that area on the shoulder. This type flame produces an unstable flame which will waiver and flutter, as well as, in the case of a vertical furnace for melting copper, causes a cold spot at that area on the shoulder at which metallic copper may deposit. This deposition of copper in the combustion chamber and/or unstable flame adversely affects the operation of the furnace causing impurities in the melted copper and possible shutdown of the furnace for cleaning. The flame unstability is particularly severe at the start-up of the burner when the furnace and burner are cold, but is also undesirably present at "steady-state" operation.
This problem has been particularly severe in the shaft furnace art when it is desired to design a furnace, e.g., one having a modest capacity, i.e., less than about 20 short tons per hour (STPH), that can be turned down over a 2.5 to 1 ratio while maintaining a stable flame in the burner. As used herein, "turndown" means the reduction of the melting rate of the furnace by decreasing the amount of fuel-oxygen mixture supplied to the burners. A turndown ratio of 2.5 to 1 for a furnace having a maximum design capacity of 20 STPH would bring the output as low as about 8 STPH, i.e., 20/2.5=8. To properly run the furnace over such a wide operating range requires the burner to maintain a stable flame over the turndown range. If the flame is unstable at the low operating rate, the melted copper will become contaminated, metal will deposit in the combustion chamber, and other problems will occur.
For example, furnaces of low capacity have traditionally operated with but a single row of circumferentially spaced burners, since a second row has been thought would furnish more molten capacity than necessary, thus causing problems in turning the furnace melting rate down to lower rates without avoiding metal "slumping" and freezing within the furnace. In small furnaces the problem of obtaining a uniform distribution of heat from a single row of burners has been a severe problem. In these one row furnaces, at maximum heating rates, the metal easily can become suspended above the burner row, causing metal hang up problems when the nonmolten charge fails to descend from the large upper diameter section into the reduced lower diameter section of the furnace. Such a phenomenon results in high oxygen levels in the metal, uneven temperatures, and furnace "screaming". Additionally, the phenomenon of "voiding", which is an absence of furnace charged metal at the furnace base, also occurs as a result of this uneven heat distribution, and leads to overheated refractories, wide variations in molten metal temperatures, and large fluctuations in metal flow therefrom when operated at or near maximum heat input. In two row furnaces, and even one row furnaces, reduced turndown over long time periods can lead to the metal becoming "soft" as the supply of fuel diminishes and, hence, heat input is reduced, leading to eventual metal slumping within the furnace.
It is an object of the invention to provide a high velocity gas burner which has enhanced flame stability over a wide operating range. Other objects will be apparent from the following description.